1. Technical Field of the Invention
The present invention relates to a nonlinear optical dye used in the technical fields of optical recording and the like and also to a photorefractive material composition, a photorefractive substrate and a hologram recording medium that are obtained using the nonlinear optical dye.
2. Description of the Related Art
Photorefractive materials are materials in which, when two light beams are crossed on or in a photorefractive material, the refractive index varies in response to a spatial electric field caused by the interference of the light beams.
Therefore, by utilizing such properties of photorefractive materials, the photorefractive materials can be applied as optical modulation elements that perform nonlinear signal processing of signal light beams. More specifically, the photorefractive materials can be used as hologram recording media utilizing formation of diffraction gratings and as optical switching elements utilizing energy transfer. The photorefractive materials can also be used as phase-conjugate mirrors because the diffracted light generated from the diffraction grating is phase-conjugate light.
Inorganic photorefractive materials have conventionally been used as such photorefractive materials. In recent years, photorefractive materials using amorphous organic compounds (referred to as “organic photorefractive materials” hereinafter) are required to be developed as substitute for the inorganic photorefractive materials because the organic photorefractive materials can be manufactured at low cost and have excellent formability and workability.
For example, Patent Document 1 (Japanese Patent Application Publication No. 2007-41324) discloses, as such an organic photorefractive material, an organic photorefractive material comprising an organic photoconductive compound, a sensitizer, a plasticizer, an electric field responsive optical function compound, and an antioxidant. However, the organic photorefractive material disclosed in the above Patent Document 1 has a problem in that the diffraction response speed is low.
A nonlinear optical dye having a specific absorption spectrum is used as a material that constitutes an organic photorefractive material. For example, Non-Patent Document 1 (Akiyoshi Tanaka et al., “Asymmetric Energy Transfer in Photorefractive Polymer Composites under Non-Electric Field,” Molecular Crystals and Liquid Crystals, Vol. 504, pp 44-51, 2009) discloses a technique of manufacturing an organic photorefractive material by using 3-[(4-nitrophenyl)azo]-9H-carbazole-9-ethanol as a nonlinear optical dye and compounding a polymer having inactive photorefractive properties as a binder material in the nonlinear optical dye. However, also in the organic photorefractive material using a nonlinear optical dye as disclosed in the above Non-Patent Document 1, the problem is that the diffraction response speed is low as in the above-described Patent Document 1.
Moreover, in the above disclosure of Non-Patent Document 1, the 3-[(4-nitrophenyl)azo]-9H-carbazole-9-ethanol used as a nonlinear optical dye has an absorption band in the visible light region and can only transmit light of wavelengths near the red light in the visible light region. Therefore, when the obtained organic photorefractive material is used as a hologram recording medium, the holographic image can be obtained only with red color, which will be problematic.